Patent GB-783854-A (GB)
Title: An improved subscription television system
IPC: H04N7/16
Filed: 1953-10-06
Abstract: 783,854. Television. SKIATRON ELECTRONICS & TELEVISION CORPORATION. Oct. 6, 1953 [Oct. 23, 1952], No. 27441/53. Class 40 (3). [Also in Groups XXXV, XXXVI, XXXVII and XL (c)] A television transmission system permitting the reception of programmes only by authorized subscribers has means for jittering the timing of the horizontal scanning so as to cause interference with reception by conventional receivers. The jittering is effected under the control of a coding device having a large number of possible circuit connections which are preset in a particular manner for each programme or period of transmission. At a receiver a similar decoding device is provided for controlling the receiver to execute a corresponding jitter, but the corresponding circuit connections can only be established by means of a decoding element in the form of a card having a series of apertures or edge slots, or bearing a printed circuit, which is obtained from the programme originators. In the particular embodiment described, the jitter is brought about by generating a number of horizontal synchronising pulses occurring at slightly different times, e.g. 2 Ásec. apart, and selecting them in a sequence to trigger the camera sweep circuit. The sequence is determined (a) by the coding device as established by the particular circuit connections set up and (b) by the form of a group of controlling voltage waves. In one arrangement the waves are generated in a regular manner so that the jitter sequence is determined solely by the circuit connections. In another arrangement the controlling waves are generated in a random manner so that the jitter sequence is also random. In order to operate the decoding device at the receiver, the controlling waves are communicated to the receiver by means of pulsed super-audible tones in the sound channel. The transmission includes also the normal correctly-timed horizontal synchronizing pulses and from these the receiver produces a group of time-spaced pulses similar to those at the transmitter. Once the corresponding circuit connections have been established by the decoding card, the decoding device operates under control of the controlling waves to select the-pulses and pass them to the cathode-ray tube sweep circuit in the same sequence as at the transmitter so that the display raster is jittered in synchronism with the camera raster and holds the picture stationary. Reference is made to the use of alternative modulations for the super-audible tones, such as frequency, frequency-shift or partial amplitude. The voltage waves may also be communicated by pulse signals in the video channel occurring during the horizontal or vertical blanking intervals. The signal may be sent over telephone or power lines. The receiver decoding circuit produces blanking waves to mask the edges of the picture spoilt by the jitter action. The invention may operate in conjunction with a coin-freed system in which, in addition to the use in the decoder of a decoding card, coins must be inserted. Alternatively the decoder may be arranged so that a hole is punched in the card indicating a programme received and a charge levied subsequently. Transmitter coding device, Fig. 2. Generators A, B and C product pairs of voltage waves as indicated in Fig. 3 under the control of vertical synchronizing pulses applied at 10. In one arrangement of generators, Fig. 5 (not shown), the voltage waves are produced in a regular manner, the generators merely comprising frequency-dividing stages arranged so that the waves at terminals 6 and 7 are half the frequency of those at terminals 4 and 5, and those at terminals 8 and 9 are half the frequency of those at terminals 6 and 7. In another arrangement, Fig. 7 (not shown), the waves appear in random manner as shown in Fig. 3 although having their transitions located by the synchronizing pulses. In this arrangement, each generator comprises a multivibrator triggered by vertical synchronizing pulses which are applied through a gate opened in a random manner under the control of a noisy pulse source such as a sporadically firing thyratron or a free running but noise-triggered multivibrator. The waves on terminals 4-9 are combined in various ways as determined by the circuit connections set up by a group 17-25 of three position switches. The combined waves appear on lines 30-32 and are applied to open gates comprised of crystal rectifiers 33-35. The waves are each clamped with the positive excursions at earth potential and the arrangement is such that a gate is opened only when all of the waves forming a combination are at earth potential. The coding device includes also a generator 12 triggered by the horizontal synchronizing pulses at 11 and arranged to produce three pulses 1, 2 and 3 on leads 13, 14 and 15 at slightly different times, e.g. 2 Ásecs. apart, as shown in Fig. 4. Pulses 1 and 2 are then applied to gates 33-35 as determined by the setting of further switches 26-28 and are transmitted through the gates to a trigger circuit 39 employed for driving the camera horizontal scan. Pulses 3 are applied continuously to the trigger circuit through crystal rectifier 37. Trigger circuit 39 produces a pulse having a width greater than the space between pulses 1, 2 and 3 and hence triggers on the first pulse received and does not recover in time to trigger on succeeding pulses. The system, therefore, causes the timing of the camera horizontal scan to be varied from field to field in a sequence as determined by the setting of switches 17-28 and which may vary in a large number of different ways. Tone generators 40, 41 and 42 operating at 20, 30 and 40 kc/s. respectively are controlled by the voltage waves on terminals 5, 7 and 9 and apply signal through mixer 43 either to the sound channel or a telephone or power line. Reference is made to the use of a matrix of diodes or gate stages for combining the voltage waves on terminals 4-9. The connections from terminals 4-9 may be transposed to introduce further variations in the pulse sequence. Receiver decoding device, Fig. 11. The detected signal in the sound channel appears at terminal 100 and is applied through a filter 150, which selects only the 20, 30 and 40 kc/s. tones, to amplifier stages 151, 156 and cathode follower 157. The tone signals are then separated by filters 159, 160; 161, 162 and 163, 167 and applied through detecting and inverting stages to produce pairs of voltage waves of leads 193, 197; 194, 198 and 195, 199 corresponding to those applied to the transmitter coding device. The waves are applied to a network of resistors, capacitors and crystal rectifiers identical with that employed in Fig. 2 but shown rearranged. Pulses corresponding to pulses 1, 2 and 3 are produced on leads 252, 253 and 254 by means of a circuit triggered by horizontal synchronizing pulses applied at 125. In this circuit a sawtooth oscillator 221 operating at horizontal scan frequency produces anode and cathode waveforms as shown at A and B in Fig. 12 and pulses C on lead 252 which constitute the pulses 1. The oscillator triggers a second similar oscillator 222 through a resonant delay circuit 234-236 producing the grid waveform D, this second oscillator producing pulses I on lead 253 to form the pulses 2 and in turn triggering a third oscillator 223 through a delay circuit 225-227. The third oscillator produces pulses J on lead 254 which form the pulses 3. The delays between the pulses are arranged to correspond with those at the transmitter by manual adjustment of the capacitors or inductors forming the delay circuits. The anode waveform from oscillator 223, curve K, is integrated and clipped, curves L and M, in a network including rectifier 536 and fed back to a network 240-246 where is combined with the horizontal synchronizing pulses M to produce the waveform 0 which is fed into a stage 220. This stage functions to provide automatic frequency control by varying the bias applied to the grid of oscillator 221 and stabilizes the circuit operation with a delay Q so that the pulses 1, 2 and 3, curve P, Fig. 12, occur before the commencement of the blanking pedestal in the received video waveform curve V, Fig. 12. Connections identical with those at the transmitter are made in the network by a decoding element (see below) by joining appropriate combinations of the terminals shown at the righthand lower side of the Fig. 11. The circuit then functions to select the pulses 1 and 2 in the same sequence as at the transmitter and apply them over lead 270 to inverter stage 275. The pulses 3 are applied directly over lead 254. The output of the inverter triggers a multivibrator 276 which produces anode, grid and cathode waveforms as shown in curves Q, S and R respectively of Fig. 12. The anode waveform at terminal 277 or inverted at 291 is employed for the cathode-ray tube blanking, and by virtue of the delay produced by the automatic control system commences before the signal blanking and is effective to obscure the fluctuations produced at the edges of the picture due to the jitter action. The grid waveform is applied to a stage 282 which in turn produces anode and cathode voltage waveforms U and T, Fig. 12. These voltages which provide either a sawtooth or pulse type of drive then control the horizontal scanning, jittering it in synchronism with the jitter at the transmitter. Decoding element. One arrangement, Fig. 13 (not shown), comprises a card similar to that employed in standard punched-card tabulating machines and provided with holes punched in vertical and horizontal columns which allow connections to be established between front and rear assemblies of contacts. Another arrangement, Fig. 14 (not shown), comprises a card having an arrangement of slots cut along one edge. This edge engages a row of spring fingers forming the movable members of an assembly of switches and depresses a c